The determination of properties of a laser beam is of interest in many technical fields. A typical area is the measurement and testing of beam properties in laser material processing systems to ensure a consistently high processing quality.
To achieve a high process quality in laser material processing systems, compliance with process and laser parameter within narrow limits is necessary. The process windows are very small especially in highly dynamic processing systems. In order to ensure the processing quality, a frequent and exact inspection of the properties of the laser beam is therefore necessary. This is particularly true for systems in which the laser beam can be freely positioned and moved by means of a scanning device in a at least two-dimensional working area, for example by deflecting the beam using movable mirrors and a subsequent scanner optics. Such remote laser processing systems have a variety of applications, such as marking, welding, cutting, and the like. A relatively new application is Selective Laser Melting (SLM). In this method, complex three-dimensional objects can be produced through local melting or sintering of successive thin layers of a powdered material. A shortening of the production time is desirable, since the production time of the objects is naturally long in such a process. To achieve this the laser beam must be moved faster, which in turn requires higher laser powers in order to melt the powder in a shorter time. Therefore, laser beams with very small focus and relatively high power are used in SLM systems, in other words, beam sources with high brilliance in which extremely high-power densities may be present in the focus of the laser beam.
Many different methods and devices suitable for this purpose are known for determining the geometrical properties of a laser beam. A first rough classification of the methods can be made according to whether a spatially resolving (e.g. pixel-based) sensor is used for the measurement, or whether the beam is scanned in a method with raster-like movement in a temporal-spatial manner.
In the first group of methods employing a spatially resolving sensor, there is always the problem that conventional pixel-based sensors do not tolerate high power densities, and therefore a beam splitting (or coupling-out or sampling), attenuation and/or imaging of the beam is always required when a beam of high power density is to be measured. However, the beam is always also influenced in its properties through the beam splitting, attenuation or imaging. For instance, optical elements such as lenses, beam splitters and attenuators can already cause thermally induced changes in the imaging due to a very slight absorption of the laser radiation. It is therefore hardly possible to ensure that the measured quantities are explicitly related to the desired quantities directly in the beam. Regarding this problem, reference is made to the publication no. DE 10 2012 106 779 A1. An optical system for measuring laser radiation is disclosed there. In the disclosed apparatus, the explicit relation of the measured quantities to the sought quantities directly in the beam can only be approximately achieved in a complex interplay of various beam splitters and imaging elements from different materials. Furthermore, the apparatus disclosed is only suitable for measuring stationary laser beams.
Other apparatuses for determining the properties of a laser beam, in which a spatially resolving sensor is used for the measurement, are briefly mentioned below only for the sake of completeness.
The publications JP H01-107 990 A (abstract) and JP 2008-264 789 A (abstract) disclose, for example, devices with location-resolving sensors which are intended for the geometrical calibration of remote systems. In these apparatuses, the light scattered by a substrate arranged in the working area is imaged onto a camera by means of a lens. From this, the position of the beam in the working area can be determined and compared with the desired position. However, such apparatuses are not suitable for measuring the focus diameter or a beam profile in the focus of the laser beam because of the achievable spatial resolution.
In the devices shown in DE 10 2007 053 632 A1 and DE 10 2011 054 941 B3, a fraction of the laser beam to be measured is reflected and thrown back into the optical system which emits the laser beam. In this case, the reflected-back beam is coupled out within the optical system and is evaluated by a location-resolving sensor. In this case, the reflection of the beam takes place, for example, at a boundary surface of the imaging optical system, typically at the last boundary surface of the focusing lens or at a subordinate protective glass. The disadvantage with these apparatuses is that the back-imaging by the focusing lens is partially loaded with considerable imaging errors, since the back-reflected beam has different dimensions and waist positions, and the correction of the focusing lens is not simultaneously designed for the imaging of back-reflections with other dimensions and waist positions. The systems of this type at the current state of technology are therefore not suitable for precise beam measurement.
The second group of methods for the determination of properties of a laser beam is characterized by a spatial-temporal scanning of the beam in a raster-like movement. This group can be divided into two subgroups according to whether the beam is sampled quasi-point-shaped or whether the type of scanning generates a signal which already contains information integrated in a spatial direction. The latter subgroup includes devices with sampling by slit apertures and knife-edge diaphragms.
Apparatuses with quasi-point-shaped sampling systems, for example by means of a measuring orifice or a measuring needle whose opening is small compared to the beam diameter, are known, for example, from DE 199 09 595 A1 and EP 0 461 730 A1. The detector is typically guided in a line-by-line scanning movement through the cross-section of the beam. In doing so, the beam must be scanned in many passages with mutually slightly shifted lines.
In the case of apparatuses for beam diagnosis in remote laser processing systems, the special feature is that the laser beam can be deflected by means of a scanning device in typically two dimensions so that the beam can be freely positioned in a planar or sometimes also three-dimensional working space. In this case, it is also possible for the sampling device to be stationary and that the beam is guided across the sampling probe by means of the scanner.
The process disclosed in DE 10 2005 038 587 A1, for example, works according to the latter principle. A measuring system is proposed therein in which, using a deflection system, a laser beam can be moved in a definable pattern across a detector arrangement. A similar method according to this principle is shown in DE 10 2011 006 553 A1. It specifies a method for determining the focal position or the beam profile of a laser beam by means of scanning optics, in which a pinhole aperture with a downstream detector is arranged at several measuring points in the working space of the laser beam. The laser beam is moved over the measuring hole of the pinhole aperture using the scanner optics according to an xy grid at each of the measuring points for an xy focus position or beam profile measurement. Another, similar device of the aforementioned type is disclosed in U.S. Pat. No. 6,501,061 B1. The laser beam is scanned across an aperture and the scanner can be position-calibrated by comparing the scanner position data at the time of the laser beam detection.
Publication No. KR 10 2013 0 121 413 A (abstract) also shows a calibration system for a laser beam scanner. There, a calibration plate is disclosed in which light-diffusing areas are formed in several points. The light reflected from the light-diffusing areas of the calibration plate is received by a light sensor part which is arranged adjacent to the calibration plate. The apparatus shown is thus not suitable for determining a beam diameter or a beam profile of a laser beam. Furthermore, it is also not possible with the device shown to exactly discriminate which of the light-diffusing points is hit by the laser beam.
The accuracy of the quasi-point-shaped scanning methods is, in principle, limited, inter alia by the size of the measuring orifice, the accuracy of the guiding movement, the synchronization of the individual lines, and not least by the reproducibility or the temporal constancy of the laser beam. A moving laser beam can thus not be measured by means of methods of this type or only under very special circumstances. In addition, the measuring orifices or measuring needles of the known devices are also only suitable for certain maximum power densities and can be destroyed in the case of highly brilliant, focused laser radiation.
A scanning can also be performed using a line-shaped sampling probe, for example with a line aperture, a knife-edge or a slit diaphragm. In the case of linear scanning, the beam intensity is already integrated in one direction. The advantage is that the beam diameter can be determined with a single scanning pass. The publications U.S. Pat. No. 5,078,491 A and JP S62-2 4117 A (abstract) disclose, for example, devices with linear scanning systems. However, even with these devices known from prior art, it is disadvantageous that the measuring orifice or knife-edge diaphragm, as well as the detectors located behind them, are suitable only for certain maximum power densities and can be destroyed in case of highly brilliant, focused laser radiation, and that the measurement of moving laser beams is not sensibly possible.
The publication no. WO 98/50 196 A1 discloses a device for detecting and calculating a focus point position, a shape, and a power distribution of a laser beam after a focusing lens. The described device includes amongst other things a light affecting body and a light sensor. The laser beam and the light affecting body are movable relative to each other, to perform a tracing movement through the laser beam. The light affecting bodies are described as optical fibers; reflective, e.g. silver-containing bodies or absorbing bodies are described as alternatives. Thus, the designs shown are on the one hand not suitable for laser radiation of highest power and brilliance, and on the other hand, the described apparatus is not suitable for achieving high spatial resolution since the disclosure does not describe an exactly defined interaction geometry on or in the light affecting bodies.
From the publication JP 2000-310 559 A (abstract) a device is known, in which a beam that is deflected by a polygon scanner is moved over optical grids with different orientations. The light which passes the grid is captured with a photoelectric converter element and the received signal is an oscillating signal, from the amplitude of which a beam diameter can be determined. The procedure shown is thus not suitable for determining the position of a beam and beam profile. Moreover, the use with high power laser radiation is limited by the power compatibility of the used grids and the photoelectric converter element.
The devices known from the state of the art with linear or knife-edge scanning thus also have the well-known disadvantages: the power compatibility is limited; the knife-edges, slit apertures or other line-shaped detection elements used can be destroyed through a focused, highly brilliant laser beam of high power. Furthermore, the majority of processes known can only determine beam dimensions in the scanning direction. In general, the achievable spatial resolution is limited, especially in scanning systems that should be suitable for higher performance. In many methods, a determination of a beam profile is not possible or only with low resolution. The determination of a lateral position of the laser beam in a field of work is also usually not possible. Moreover, the measurement of a moving laser beam is not possible or only in special cases with preset laser beam control.
The apparatuses and methods known from state of the art thus have serious disadvantages both with regard to the achievable accuracy and spatial resolution, as well as with regard to the compatibility with laser beams with high power and finally with regard to the measurement of laser radiation in large working areas or in case of moving laser beams.